The vast majority of combustion processes use air as an oxidizer in combustion with a fuel such as natural gas, fuel oil, propane, waste oils, other hydrocarbons and the like. It is also known that the performance of many air-fuel combustion processes can be improved by enriching the combustion air with oxygen. Enrichment of the combustion air increases both the flame temperature and the thermal efficiency while the flue gas volume decreases as the oxygen concentration in the air or oxidizer increases. The cost of high purity oxygen for enrichment can be offset by gains in productivity from the enhanced combustion. Low level enrichment of up to 35% total oxygen content in the oxidizer can generally be retrofitted to existing air-fuel systems with few modifications.
Recently, the environmental impact of combustion processes has received a great deal of attention. It has been shown that the nitrogen oxides, known as NOx, are detrimental to the environment by producing smog, acid rain, and ozone in the lower atmosphere, the latter being a cause of global warming. In the United States the new Clean Air Act signifies the commitment of the Federal Government toward controlling pollution. The new regulations have spurred an increased interest in controlling NOx formation as a result of combusting air-fuel mixtures.
It is also known that low level oxygen enrichment in combustion can cause a dramatic increase in NOx emissions. In industrial combustion processes, over 90% of the NOx emissions are in the form of a nitric oxide or NO. It has also been shown that at high levels of oxygen enrichment, e.g. above 90% total oxygen content in the oxidizer, less NOx is produced than using air for the same firing rate. However, high levels of oxygen enrichment can be uneconomical in a given process and in fact may produce materials problems also.
In the past, two strategies have been used to reduce NOx emissions. The first is to remove the NOx from the exhaust gases before they exit into the atmosphere. Post-treatment of the exhaust gases from the combustion process can be carried out by selective catalytic or non-catalytic reduction and various combinations of oxidation, absorption and reduction processes to scrub out the NOx. These processes generally involve high cost and a need to shut down the combustion process in the event of failure with the post-treatment equipment. The second method or strategy is to minimize NOx formation in the first place by modifying the combustion process in some manner. Examples of this second strategy include water or steam injection into the flame, reduced excess air in the combustion process and so-called low NOx burner designs. These technologies provide the user with a penalty in reduced thermal efficiency or only minimal NOx reductions.